Studies will be carried out on the mechanisms by which polyoma virus's middle T antigen transforms cultured fibroblasts. Since middle T has no known biochemical activity it is thought to transform by binding to and modulating the activities of certain key cellular proteins. Hence these studies will focus on two complementary aspects of middle T antigen: First a series of genetic studies will be carried out on middle T. These will be aimed at determining how many key sites on the molecule are important to transformation. As sites are defined they will be tested to see into which complementation group they fall. Additional genetic studies will be carried out on two known motifs as well. Biochemical studies will focus on Mtag's interaction with the Pi3' kinase and the 27/29kDa cellular proteins which are bound by Mtag and on identifying cellular proteins which are phosphorylated on tyrosine in Mtag transformed cells.